Device for measuring the mass of a flowing medium

ABSTRACT

A device for measuring the mass of a flowing medium, or flow rate meters, having a measuring element accommodated in a measurement support; upstream of the measuring element, a grid with flow openings is provided. To attain a substantially rotational symmetrical velocity distribution even in the case of a severely impeded oncoming flow, the grid has flow openings, which at least regionally have a different flow cross section. The flow cross sections of the flow openings are adapted to the oncoming flow, in order to bring about a flow with substantially even velocity distribution downstream of the grid. The device is intended for measuring the mass of a flowing medium, in particular for measuring the mass of aspirated air in internal combustion engines.

BACKGROUND OF THE INVENTION

The invention is based on a device for measuring the mass of a flowingmedium, also known as a flow rate meter. A device is already known(European Patent 0 458 998) that has a measuring element, accommodatedin a measurement support, and a flow rectifier and a grid accommodatedupstream of the measuring element. The flow rectifier is provided inorder to generate as uniform a flow as possible over the entire insidecross section. The grid, permanently secured to the flow rectifier, isintended to create superfine eddies in the flow, in order to create themost constant possible flow conditions downstream of the grid, therebystabilizing the measurement signal at the measuring element. The grid isa wire grid, which has individual wires woven into a meshlike structure.The wire grid has equidistant mesh widths, so that there are many flowopenings all having the same flow cross section. If there is a severelyimpeded oncoming flow to the grid, which is characterized by anonuniform distribution of velocity and major velocity gradients,however, the result is an uneven distribution of velocity, evendownstream of the grid. Such a velocity distribution has adisadvantageous effect on the measurement accuracy of the measuringelement, however.

In the known device, the grid is embedded in the heated state in a ringof the flow rectifier. Since the grid is made up of individual wiresthat are woven together into a mesh structure, the wires can shiftsomewhat relative to one another. Embedding the wire grid in the plastichas the disadvantage that a temperature change and/or aging of theplastic of which the flow rectifier is made can cause the wire grid tobecome dented or to sag. If the wire grid becomes dented or sags, theindividual wires of the grid shift about, disadvantageously altering thecharacteristic curve of the measuring element. Moreover, securing thegrid to the flow rectifier permanently has the disadvantage that onlyrelatively complicated flow rectifiers, with grids of variable meshwidth, can be combined with one another. There is also the risk thatwhen the grid is embedded in the heated state in the ring of the flowrectifier, expelled plastic will remain in the flow rectifier, creatingobstacles in the flow that can cause signal scattering, especially inmass production. Moreover, the provided embodiment of a ring thatprotrudes from a surface of the flow rectifier disposed at right anglesto the flow direction, is relatively complicated from a productionstandpoint.

Another known option for securing the wire grid is to provide the wiregrid with a flanged-over edge that has notches in which ribs can engagein order to position the wire grid. The mounting or flanging over of theedge of the wire grid is complicated from a production standpoint, onthe one hand. On the other, the notches provided on the edge, because ofunavoidable production variations, on being flanged over can beassociated only relatively imprecisely with the flow openings of thewire grid, and accordingly a precise alignment of the flow openings ofthe wire grid with the openings of the flow rectifier is impossible.

OBJECT AND SUMMARY OF THE INVENTION

The flow rate meter according to the invention has the advantage overthe prior art that even in the event of a severely impeded oncoming flowwith an uneven distribution of velocity in the measurement support, aprecise measurement outcome is established at the measuring element. Itis especially advantageous that unavoidable installation tolerances ofthe measuring element in the measurement support now have hardly anyinfluence on the measurement accuracy of the measuring element. It isalso especially advantageous that certain flow regions, for instanceregions involving heavy soiling, can be excluded or screened from theflow by means of the grid, so as to counteract deposits on the measuringelement.

Advantageous further features and improvements to the device disclosedare possible with the provisions recited herein.

It is also advantageous that in an especially simple way, grids ofvariable mesh width, for instance with various flow cross sections, canbe made. It is especially advantageous that sagging or denting of thegrid can be prevented, which increases the measurement accuracy and inparticular the measurement stability over a long-term operation of thedevice. It is also advantageous that grids with a variable mesh width orvariable flow cross sections can be produced without special expenditurefor tools. For an intended dismantling of the device, the flow rectifierand the grid are individually present and can then be easily separatedfor recycling. The contemplated mode of producing the grid by punchingis especially advantageous, because it allows producing the flow crosssections of the grid with very good accuracy, yet without increasing theproduction cost for the grid.

The invention will be better understood and further objects andadvantages thereof will become more apparent from the ensuing detaileddescription of preferred embodiments taken in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows how a grid embodied according to theinvention acts upon the flow;

FIG. 2 shows a device, equipped with the grid embodied according to theinvention, in a fragmentary sectional view;

FIG. 3 is a plan view of the grid in a first exemplary embodiment of theinvention;

FIG. 4 is a plan view of the grid in a second exemplary embodiment ofthe invention;

FIG. 5 is a plan view of the grid in a third exemplary embodiment of theinvention;

FIG. 6 is a plan view of the grid in a fourth exemplary embodiment ofthe invention; and

FIG. 7 is a plan view of the grid in a fifth exemplary embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1 and 2, a device 1 for measuring the mass of a flowing medium,in particular the aspirated air mass of internal combustion engines, isshown in cross section. The engine may be a mixture-compressing enginewith externally supply ignition, or an air-compressing, self-ignitingengine. The device 1 has a measurement part 2 that is inserted, forinstance in plug-in fashion, into a measurement support 5 of thedevice 1. The measurement part 2 has a slender, barlike, parallelepipedshape extending longitudinally of an insertion axis 10, and isintroduced, for instance in plug-in fashion, into an opening made in awall 8 of the measurement support 5. The wall 8 defines a flow crosssection, which for example has a circular cross section, in the middleof which a center axis 11 extends parallel to the wall 8, this axisbeing oriented at right angles to the insertion axis 10. A measuringelement 14 is introduced with the measurement part 2 into the flowingmedium. A measurement conduit 15, in which the measuring element 14 formeasuring the medium flowing in the measurement support 5 isaccommodated, is embodied in the measurement part 2 of the device 1. Thestructure of such a measurement part 2 with a measuring element 14 isadequately well known to one skilled in the art, for instance fromGerman Offenlegungsschrift DE-OS 44 07 209, U.S. Ser. No. 545,583 filedNov. 3, 1995, whose disclosure is hereby incorporated by reference.

Upstream of the measurement part 2, a grid 21 is provided, as shown inFIG. 1 is intended to compensate for more or less major nonuniformitiesin the velocity distribution in the oncoming flow to the measuringelement 14. Such nonuniformities in the velocity distribution, indicatedby corresponding velocity arrows 16 in FIG. 1, are due to flowobstacles, such as an elbow, not shown, provided upstream of the grid21, or an air filter. By the embodiment of the grid 21, according to theinvention, the intent is, at least in the region of the measurement part2, to generate a substantially rotationally symmetrical velocitydistribution, represented by velocity arrows 17 in FIG. 1, that has nomajor velocity gradients, so as to achieve a precise measurement resultat the measuring element 14.

As shown in FIG. 2, besides the grid 21 a flow rectifier 20 may also beprovided, which is accommodated upstream of the grid 21 in themeasurement support 5. By way of example, the flow rectifier 20comprises plastic and is made by injection molding and has many openings24, for instance rectangular in shape. For mounting purposes, the grid21 is introduced into an opening 23, for instance round in shape,provided on the upstream end of the measurement support 5, until withits back face 26 it rests on a stop 25 that reduces the size of thecross section of the opening 23. Next, the flow rectifier 20 can beinserted into the opening 23, until it rests on spring elements 30provided on the grid 21. For permanent securing of the flow rectifier 20in the opening 23, the flow rectifier 20 has barblike detent hooks 33,protruding approximately radially outward from its outer face 22, forinstance, which can lock into a groove 25 provided in an inner wall 34of the opening 23. As the flow rectifier 20 is introduced into theopening 23, the spring elements 30 of the grid 21 are elasticallydeformed and exert an axially oriented spring force on the flowrectifier 20. Upon reaching the installed position of the flow rectifier20 in the opening 23, the detent hooks 33 lock into the groove 35, andwith the aid of the spring force of the spring elements 30 theypermanently retain the flow rectifier 20 and the grid 21 in the opening23 under axial tension. A detent ring encompassing the circumference ofthe flow rectifier 20 may also act as the detent hooks 33. The springelements 30 not only can be embodied on the grid 21 in such a way thatthey rest on the flow rectifier 20 but can also rest on the stop 25 orcan rest in alternation on the flow rectifier 20 and the stop 25.

The radial orientation of the grid 21 with regard to the openings 24 inthe flow rectifier 20 is effected by means of at least one notch 37,shown in FIG. 3, provided on one edge 44 of the grid 21. The at leastone notch 37 extends radially inward from the edge 44 and has arectangular shape, for instance, in order to receive a correspondinglyformed rib 38 extending radially inward from the inner wall 34 of theopening 23 of the measurement support 5. The notch 37 is recessed with aprecisely predeterminable location relative to the flow openings 40 ofthe grid 21, so as to align the flow openings 40 precisely with theopenings 24 of the flow rectifier 20. The location of the at least onenotch 37 is unequivocally defined relative to the flow openings 40 ofthe grid 21. The at least one notch 37, like the grid 21 and the flowopenings 40, can be made by being punched out of a thin metal strip.Particularly in mass production, in a simple way this produces a veryexact alignment and association of the flow openings 40 of the grid 21with regard to the notch 37 and hence to the openings 24 of the flowrectifier 20.

According to the invention, the grid 21 has a grid structure, which iscomposed of a plurality of flow openings 40 in such a way that at leastregionally, various-sized flow cross sections are present. A firstexemplary embodiment of such a grid 21 is shown in more detail in FIG.3, which is a plan view on the grid 21. The grid 21 is circular in itsouter shape, for instance, and by way of example has many rectangularflow openings 40, which decrease in size from a middle region 42 in thecenter of the grid 21 toward the edge 44, so that in an outer region 43of the grid 21, there are fine-pore flow openings 40, while in themiddle these openings have large pores. The refining of the grid 21 fromthe middle region 42 toward the edge 44, in the event of an unevenvelocity distribution with high speeds in the vicinity of the wall,enables a corresponding compensation, so that at a certain distance fromthe grid 21 a rotationally symmetrical velocity distribution prevails,which is characterized by a substantially constant velocity distributionover the flow cross section.

However, it is also possible, as shown in FIG. 4 for a second exemplaryembodiment of the grid 21, to embody the grid 21 with smaller ornarrower-mesh flow openings 40 in the middle region 42 and larger orcoarser-mesh flow openings 40 in the region of the edge 44. This kind ofstructure of the grid 21 makes it possible to purposefully reducevelocity spikes in the region of the middle axis 11 in the oncoming flowthrough the narrower-mesh flow openings 40, so that once again a uniformvelocity distribution over the flow cross section, or a constantvelocity downstream of the grid 21, results.

As shown in FIG. 5 for a third exemplary embodiment of the grid 21 ofthe invention, it is also possible for only very specific regions of theflow, for instance heavily soiled regions, to be faded out by refiningthe grid 21, specifically by providing substantially narrower flowopenings 46 there. The flow cross sections of the flow openings 40 mayfor instance be embodied as triangular, quadrangular, pentagonal,hexagonal, or many-sided, or round or oval.

However, it is also conceivable, as shown in a fourth exemplaryembodiment of the invention in FIG. 6, to make the flow openings 40asymmetrical, for instance in beadlike form toward the edge 44, withincreasing distortion.

It is also possible, as shown in FIG. 7 for a fifth exemplary embodimentaccording to the invention of the grid 21, to provide many circular flowopenings 40, for instance, which have a narrow flow cross section in themiddle region 42 and a greater flow cross section toward the edge 44,for instance. However, it is also conceivable for the flowing openings40 to have a smaller flow cross section in the outer region 43 and alarger flow cross section toward the middle region 42.

The production of the flow openings 40 in the grid 21 can be done bypunching them out of a thin metal strip. However, it is also possible tomake the flow openings 40 using a laser. It is also conceivable tosubstitute ceramic for the grid 21, for instance in order to obtain theflow openings 40 by etching them out of the ceramic substrate.

The foregoing relates to preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

We claim:
 1. A device for measuring the mass of a flowing medium,comprising a measurement support, a measuring element and a gridprovided upstream of the measuring element, the grid having many flowopenings, in which the flow openings (40) at least regionally have avariable flow cross section, the grid (21) has at least one notch (37)on an edge (44) that has a predetermined location relative to the flowopenings (40), and the at least one notch (37) receives at least one rib(38) that extends radially inward from an inner wall (34) of an opening(23) in the measurement support (5).
 2. A device in accordance withclaim 1, in which the flow cross sections of the flow openings (40) areadapted to an oncoming flow to the grid (21) in such a way thatdownstream of the grid (21), a flow with an essentially uniform velocitydistribution prevails.
 3. A device in accordance with claim 1, in whicha flow rectifier (20) is provided upstream of the grid (21).
 4. A devicein accordance with claim 2, in which a flow rectifier (20) is providedupstream of the grid (21).
 5. A device in accordance with claim 1, inwhich the flow cross sections of the flow openings (40) have arectangular or beadlike or round or oval shape.
 6. A device inaccordance with claim 1, in which the grid (21) has flow openings (40),which have a small flow cross section in a middle region (42) of thegrid (21) and an increasingly large flow cross section toward an outerregion (43) of the grid (21).
 7. A device in accordance with claim 2, inwhich the grid (21) has flow openings (40), which have a small flowcross section in a middle region (42) of the grid (21) and anincreasingly large flow cross section toward an outer region (43) of thegrid (21).
 8. A device in accordance with claim 3, in which the grid(21) has flow openings (40), which have a small flow cross section in amiddle region (42) of the grid (21) and an increasingly large flow crosssection toward an outer region (43) of the grid (21).
 9. A device inaccordance with claim 4, in which the grid (21) has flow openings (40),which have a small flow cross section in a middle region (42) of thegrid (21) and an increasingly large flow cross section toward an outerregion (43) of the grid (21).
 10. A device in accordance with claim 5,in which the grid (21) has flow openings (40), which have a small flowcross section in a middle region (42) of the grid (21) and anincreasingly large flow cross section toward an outer region (43) of thegrid (21).
 11. A device in accordance with claim 1, in which the grid(21) has flow openings (40), which have a large flow cross section in amiddle region (42) of the grid (21) and an decreasingly large flow crosssection toward an outer region (43) of the grid (21).
 12. A device inaccordance with claim 1, in which the grid (21) has a defined region(46) with a small flow cross section.
 13. A device in accordance withclaim 1, in which the flow openings (40) are produced by being punchedout of a thin metal strip.
 14. A device in accordance with claim 13, inwhich the grid (21), on an edge (44), has at least one notch (37) thathas a predetermined location relative to the flow openings (40).
 15. Adevice in accordance with claim 14, in which the at least one notch (37)receives at least one rib (38) extending radially inward from an innerwall (34) of an opening (23) in the measurement support (5).
 16. Adevice in accordance with claim 1, in which the flow openings (40) areproduced by means of a laser.
 17. A device in accordance with claim 1,in which the flow openings (40) are produced by being etched out of aceramic substrate.
 18. A device for measuring the mass of a flowingmedium, comprising a measurement support, a measuring element and a grid(21) provided upstream of the measuring element, the grid having manyflow openings which are punched out of a thin strip of metal, in whichthe flow openings (40) at least regionally have a variable flow crosssection, the grid (21) has at least one notch (37) on an edge (44) thathas a predetermined location relative to the flow openings (40), and theat least one notch (37) receives at least one rib (38) that extendsradially inward from an inner wall (34) of an opening (23) in themeasurement support (5).